It is known to provide exercise machines with motor driven cranks which are engaged by hands and/or feet of the user. Such machines may be used passively, as to provide movement of the arms and/or legs of a person incapacitated in some way, or actively where the user will try to advance or retard the rotation of the cranks, building muscle. Previous machines have cranks which are fixed in length or have a manual adjustment that can be changed only when stopped, and which are connected with a drive motor through chains, belts, and/or gears that are noisy, risk injury to the user and require guards or shields.